Include LICENSE badge in README files

[jabalde]

As a developer
I want to show the project license in the README file
so everybody knows it

As analyzed in #5, we can show license badge in README files this way:

[![License: MPL 2.0](https://img.shields.io/badge/License-MPL%202.0-brightgreen.svg)](https://github.com/corunet/openapi-nodejs-cli/blob/main/LICENSE)

Acceptance criteria

• The readme includes license badge